Are magnetite spherules capable of carrying stable magnetizations?

Abstract

To determine if magnetite spherules extracted from ancient carbonate rocks can carry ancient remanent magnetizations, electron microscope observations and magnetic data have been obtained for more than 100 individual spherules extracted from the Jurassic Twin Creek limestone, Wyoming, and the Mississippian Leadville limestone, Colorado. Two categories of surface textures (smooth and patterned) correlate with different hysteresis parameters. Spherules with patterned surfaces (Group A) have ratios of saturation remanent magnetization to saturation magnetization (Msr/Ms) < 0.1 and ratios of remanent coercive force to coercive force (Hcr/Hc) mostly > 3. Most spherules with smooth surfaces (Group B) have 0.1 < Msr/Ms < 0.3 and Hcr/Hc< 3. The patterned surfaces consist of (1) parallel dendritic nets with units ≈ 1 μm wide, (2) platelets with widths of 1 to 3 μm, (3) mosaic surface units with diameters between 2 and 5 μm; each of these surface elements consists of a single crystal, as verified by selected‐area electron diffraction patterns showing sharp spots, whereas the spherules as a whole show multicrystalline patterns by X‐ray diffraction. The Msr/Ms and Hc values of group A increase with decrease in grain size; these Group A magnetite spherules are undoubtedly multi‐domain in character and are unlikely to carry stable ancient remanences. The smooth‐surfaced Group B spherules are not very abundant (< 8% of the entire population) and have pseudo‐single‐domain (PSD) or multi‐domain (MD) magnetic properties; they may carry ancient remanences, although we infer that single‐domain (SD), non‐spherical, submicrometer magnetite is the principal carrier of remanence in most carbonates.